The mythical Yeti spawn 
Based on my research and personal experience, I would say that It is NOT possible to block the spawn.
As I have been in the hunt of this mythical creature (and the Bunny Slippers that may come with it) for years, I can share with you my very own thoughts:
My journey to find the Yeti started in 2009, I used to travel around 2-3 times per week all the way to Folda to try my luck (I used the explore the whole island, including caves, trying to find it)
In the past, it was really common to block the spawn of monsters all across Tibia, this made it "easier" for teams to complete quests such as Pits of Inferno and Inquisition, where the main team would go killing and clearing all the way while leaving behind characters just sitting in a certain sqm so the rest of the team could just go through without any risk and preventing the "back spawn" (when monsters appear behind you and you have to kill the way back as well).
In the Winter Update 10.70 in December 2014, everything changed and the spawn mechanics were improved; there was a news ticker released along with the update with Additional Changes Major Game Update 10.70 where it was stated the adjustment of the respawn behaviour of several monsters.
ogether with the release of this year's winter update, the respawn behaviour of several monsters has been adjusted.
It is not possible anymore to block the respawn of the following creatures:
...
Source: https://www.tibia.com/news/?subtopic=newsarchive
As expected, the Yeti was not included in the announcement but the Crustacea Gigantica was on the list, which kept me thinking that the same behaviour would apply to the "rare raids". I also had in mind the mechanics of raids such as Ferumbras and Orshabaal, where it was tested and it is of common knowledge that even if people are standing next to the "spawn spot", this would not prevent them of spawning.
Due to various factors in Real-Life, I had to stop playing Tibia back in 2015 and with this, my quest of finding the Yeti had a hard stop with zero encounters.
And then March 2020 came, and with it the lockdown in my country, this helped me to return to Tibia and my quest of finding the Mythical Yeti restarted. This time I used more tools for my research.
First I read as much as possible from the Yeti in various fansites to learn more of the Myth such as Tibiopedia - Yeti, and when I got to TibiaWiki - Yeti I found evidence of the drop of Bunnyslippers and a full Yeti Bestiary (Bestiary is new for me by the way, it was included in the Winter Update 2017), Disclaimer: you should take in consideration that the screenshots are from a Test Server, pending to be confirmed in a real server.
Yeti Bestiary found on: tibia.fandom.com/wiki/Yeti?file=Yeti_Bestiary_Entry.png
Nevertheless the Tibia Fansite that helped me the most it was Tibiabosses, here I could find the most useful information: an approximation of '"When" and "Where".
This time I created a low level character to keep it logged in Folda, and like my self, there are many others who do the same so by keeping track of whether a Yeti has died or killed someone, I check regularly my world either on Tibia's Kill Statistics and filtering by my world, or on Tibiabosses.com Boss Stats. Whenever there is a "high chance" of the Yeti spawning (18-25 days after the last spawn) I leave my low level char semi-afk HERE (thanks to the home office opportunity I have during the lockdown)
Map found on: https://tibiamaps.io/map#32014,31595,7:2
Since I can't be online doing this technique for 24h, several tries failed, Yeti's where killed the same days I would be AFK'ing there but in different times until last November. I left my character AFK and returned to move it around 13 minutes after it would automatically log out and FINALLY! I was killed by a Yeti while standing there!
A rush of adrenaline filled my body because I was going to face, after 11+ years, the dreaded Yeti. My emotions where high but so my nerves, it already passed 5 minutes since I died and I was thinking someone else must have got it killed already but happily I got there with my main character and got to kill it! I was so excited and nervous that I forgot to take a screenshot but I took one with my phone of the bestiary to let all my friends know I finally did it.
Image uploaded by myself in tibiagallery.com
Even though I only looted some Snowballs and Hams, for me, it confirms my technique and answers your question, the spawn of the Yeti is not blocked if you're standing there.
PS. I have used the same technique waiting for the Orc Warlord raid that may or may not give the Amazon Set. I've already died on low level afk char twice. It's harder the "When" in this one but I'm persistent.
No rares looted so far... 
Sources:
tibia.com
tibiopedia.pl/monsters/Yeti
www.tibiabosses.com/gladera/
tibiamaps.io/map#32014,31595,7:2
https://tibiagallery.com/image/img-20201113-wa0002.ECDIT
tibia.fandom.com/wiki/Yeti
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