If the "junk" is kernel messages (blue/white) about ugen2 detaching/attaching, you can either try to disable the touchscreen in BIOS, or, try disabling ugen(4) devices via boot_config(8). Example:
Code:
boot> -c
booting hd0a:/bsd: 10594072+2454544+335872+0+671744 [678428+128+854136+595475]=0xf70d30
entry point at 0x1001000
[ using 2129200 bytes of bsd ELF symbol table ]
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Copyright (c) 1995-2019 OpenBSD. All rights reserved. https://www.OpenBSD.org
OpenBSD 6.5 (GENERIC) #3: Sat Apr 13 14:42:43 MDT 2019
deraadt@amd64.openbsd.org:/usr/src/sys/arch/amd64/compile/GENERIC
real mem = 4278181888 (4079MB)
avail mem = 4139008000 (3947MB)
User Kernel Config
UKC> disable ugen
296 ugen* disabled
UKC> quit
Continuing...
mpath0 at root
scsibus0 at mpath0: 256 targets
mainbus0 at root
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